Atmospheric air is a gas mixture which is substantially composed of nitrogen (78%), oxygen (21%) and argon (0.9%). The remaining 0.1% primarily comprises carbon dioxide together with the noble gases neon, helium, krypton and xenon as further components.
Plants for air fractionation by rectification (hereinafter “air fractionation plants” for short) are known. They are used for producing gaseous oxygen and nitrogen and optionally liquid oxygen, liquid nitrogen and the stated noble gases. Air fractionation comprises the essential steps of compression, precooling, purification, cooling and rectification.
Compression proceeds for example in multistage turbocompressors with intermediate cooling and post-cooling to a pressure of approx. 6 bar or above. Prior to compression, dust particles may be removed in “intensive” filters.
Subsequent precooling may be performed in water-operated direct-contact coolers, in which water-soluble impurities may in part be washed out. The water used may, for example, be recooled in evaporative trickle coolers against residual gaseous nitrogen from rectification (hereinafter also denoted “cooled nitrogen”).
The precooled air is generally purified in molecular sieve absorbers, in which moisture, carbon dioxide and hydrocarbons are removed.
The air purified in this manner is liquefied by being cooled to approx. −175° C. in one or more main heat exchangers. Cooling proceeds by internal heat exchange countercurrently to the cold gas streams produced in the plant. In this case too, at least residual gaseous nitrogen from rectification is generally used. On subsequent expansion, the air cools further due to the Joule-Thomson effect and liquefies.
The actual fractionation (rectification) of the air proceeds in separation columns (rectification columns) of a separation column system, an oxygen-rich bottoms fraction and a nitrogen-rich overhead fraction initially being produced. Depending on the requisite purity of the final products and/or on the gases to be produced, different column configurations may be used for the separation column system. For example, two separation columns may be used as double columns in the form of “medium pressure” and “low-pressure” columns. Noble gases such as argon and/or neon may be produced by downstream separation columns and method steps. Rectification may also for example involve liquefaction of pure nitrogen against vaporizing oxygen and recycling thereof into the separation column system. Corresponding plants may also comprise further apparatuses such as for example additional or post-compressors, expansion turbines, high-pressure heat exchangers, internal compression pumps and/or liquid separators.
Air fractionation plants are thus made up of a “warm” part, which contains the components for compression, precooling and purification, and a “cold” part, which contains the main heat exchanger(s) and optionally further heat exchangers, for example, a countercurrent supercooler, and the separation column system. The components in the cold part may be arranged in one or more “cold boxes”. These are jacketed steel frames which are filled with insulating material such as perlite in order to reduce input of heat from the surroundings. The interior of a cold box is ideally maintenance free. Components which require maintenance may to this end be sealed off from the insulating material and be arranged to be accessible from the outside. Valves may extend towards the outside in order for example to make the drives thereof accessible (i.e., the valves are near the wall of the cold box so that their drives can extend through the cold box). Moisture penetration may be prevented by flushing the interior of the cold box with nitrogen.
Depending on the size of the plant, a plurality of components may be integrated in a common cold box. In relatively small plants, for example, the main heat exchanger(s) and the separation column system may be combined in one cold box, while in larger plants these components are distributed between a plurality of cold boxes. Large plants may also comprise a plurality of main heat exchangers which are accommodated in separate cold boxes. Further cold boxes, for example a plurality of column boxes and/or “argon boxes” (in plants for obtaining argon) may also be provided.
Gaseous oxygen and nitrogen obtained in an air fractionation plant may be fed into a pipework system and delivered directly to the consumer. Oxygen, nitrogen and argon in liquid form are held in intermediate storage for example in storage tanks and transported to the site of use in tankers.
Corresponding air fractionation plants should preferably be present at the site of use for the respective gases, thus for example in the vicinity of refineries or petroleum deposits in order to keep transport distances for the stated fluids as short as possible.
Air fractionation plants are here generally assembled from prefabricated components. This is, however, frequently problematic as sufficiently skilled assembly personnel are either not available or are costly. This in particular applies to linking of the main heat exchangers. There is therefore a need for improvements which enable more reliable and simpler erection of air fractionation plants.